Basic Principles of Batteries and Electrochemical Cells
Batteries are divided into two principal types, primary batteries and secondary batteries. Primary batteries are used once and are then exhausted. Secondary batteries are also often called rechargeable batteries because, after use, they may be recharged, then used again. In rechargeable batteries, each charge/discharge process is called a cycle. Rechargeable batteries eventually reach an end of their usable life, but typically only after many charge/discharge cycles.
A rechargeable battery includes an electrochemical cell and optionally other materials, such as a casing to protect the cell and wires or other connectors to allow the battery to interface with the outside world. An electrochemical cell includes two electrodes, a positive electrode, called the cathode and, a negative electrode, called the anode, an insulator separating the electrodes so the battery does not short out, and an electrolyte that transports the ionic component of the chemical reaction between the two electrodes and forces the electronic component to be transported outside the cell. The anode is the reductant of the chemical reaction, the cathode the oxidant, so on discharge electrons flow from the anode to the cathode and are charge-compensated by cations flowing inside the cell from the anode to the cathode. This process transforms the chemical energy of the reaction into electric power in the external circuit by delivering a current at a voltage for a time Δt until the chemical reaction is completed. If the charged cell has the electric current cut off, which is called open-circuit, electrons cannot flow, but the ions inside the cell can flow without being charge-compensated. As a result, the cathode becomes positively charged on open-circuit, which is why the cathode is called the positive electrode.
The cation that is transported between the electrodes by the electrolyte is called the “working ion.” A rechargeable battery is named after the working cation. For example, the positive ion in a lithium secondary battery is the lithium ion (Li+). In a sodium secondary battery it is the sodium ion (Na+).
To recharge the battery, the same process happens in reverse by the application of electric power. By supplying electric energy to the battery, electrons are induced to leave the cathode and enter the anode. To keep the overall charge neutral in the cathode and anode, a positive ion leaves the cathode and enters the electrolyte, and a positive ion also leaves the electrolyte and enters the anode. The efficiency of electrical-energy storage in a rechargeable battery depends on the reversibility of the chemical reaction between the two electrodes.
Because the ionic conductivity in the electrolyte is many times smaller than the electronic conductivity in the electrode, a battery has large-area electrodes that are separated by a thin electrolyte. Therefore, the electrodes do not need to be thick, and, their electronic conductivity does not need to be high so long as they make contact with a metallic current collector. Consequently, in addition to containing an active material that exchanges electrons and ions, anodes and cathodes may contain other materials in addition to a metal backing to which a slurry of the active material is applied and dried. The slurry often contains, in addition to the active material, a binder to help it adhere to the backing and conductive materials, such as carbon particles. Once the slurry dries, it forms a coating on the metal backing.
Several important properties of rechargeable batteries include energy density, power density, capacity, particularly reversible capacity, rate capability, cycle life, thermal stability, cost, and safety. All of these properties are influenced by the choice of materials used to form the battery. The capacity of a battery is the amount of electronic charge that is transported at a constant current between the electrodes per unit weigh in the time Δt for a complete discharge, and the energy density is the product of the average voltage during discharge and the capacity. Both decrease with increasing current and, therefore, power delivered. Moreover, the cycle life of a rechargeable battery is defined as the number of charge/discharge cycles before the capacity fades to 80% of its original capacity. Capacity fade is caused by a loss of the reversibility of the chemical reaction between the electrodes. For instance, many rechargeable sodium batteries experience a loss in reversible capacity as they are cycled because sodium ions (Na+) tend to be trapped at the surface of a strongly reducing anode and then to remain there as a passivating solid electrolyte interface (SEI) layer, making them unavailable to enter and exit the cathode and anode. Since energy density is proportional to the capacity of a battery, this loss in reversible capacity also decreases the rechargeable battery's energy density with an increasing number of cycles.
Lithium batteries also experience decreases in important parameters due to lithium ion (Li+). Extra lithium has previously been introduced into rechargeable lithium batteries to attempt to reduce reversible capacity and energy density loss. In one such battery, lithium metal was deposited directly onto the anode, but this increased manufacturing costs and deteriorated the uniformity and mechanical stability of the anode, causing other problems. In another battery, Li2MoO3 was added to the cathode, but this material has a practical capacity of only 250 mAh/g and thus was even worse at contributing charging capacity than Li2NiO2. In addition, the molybdenum (Mo) in Li2MoO3 dissolved in the electrolyte during battery use, also causing other problems.
In still another battery, Li2NiO2 was added to the cathode, but this material has a practical capacity of only 400 mAh/g, and this did not contribute sufficient capacity to be useful.
None of these materials work particularly well even in the context of lithium batteries. In addition, there is typically no reason to attempt to use counterpart materials in sodium batteries due to differences in the chemistry, crystal structure, or electrochemical properties. For instance, Li2NiO2 has a crystal structure in the space group Immm (FIG. 1A). In this crystal, NiO4/2 units share edges along the a-axis, and lithium ion (Li+) tetrahedral sites share edges in the ab plane. Li extraction causes the structure to collapse into an amorphous form. In contrast, Na2NiO2, despite the deceptively small change in the chemical formula, has a crystal structure in a different space group, Cmc21 (FIG. 1B). In this crystal, NiO4/2 units also share edges along the a-axis, but they have a different orientation as compared to those in Li2NiO2. Furthermore, the sodium ions (Na+) do not all occupy the same site in the crystal. Instead, there are two sodium ions (Na+) sites. The Na1 site is a square pyramid site, while the Na2 site is a tetrahedral site. The Na1 sites share edges with one another and also share two edges with 2[NiO4/2] units. Na2 sites share corners with one another and share one edge with a NiO4/2 unit. Crystal structure is frequently important to the electrochemical activity of a material and thus one would not expect Na2NiO2 to be usable in a sodium battery based upon the limited usability of La2NiO2 in a lithium battery.
Furthermore, Li2NiO2 collapses when first delithiated to an amorphous material that cannot reintercalate lithium, making it a one-time-only cation donor. Although Na2NiO2 has also been used in lithium batteries, it was also used as a one-time-only cation donor (Na+ cations can substitute for Li+ cations in the electrolyte) likely based on the assumption that it also collapses into an amorphous material after desodiation.
In addition, even if one considered Na2NiO2 as a potential additive to sodium batteries, it is known that delitiation of Li2NiO2 produces oxygen gas (O2) as a by-product. Sodium metal (Na) is highly reactive with oxygen gas (O2). As a result, one would have expected desodiated Na2NiO2 to produce oxygen gas (O2), which would poison the anode by reaction with sodium metal (Na) at the anode.